1. Technical Field
This invention relates to elevators in general, and more specifically to elevators having more than one car.
2. Background Information
Elevators typically consist of an elevator car and a counterweight for travel within a hoistway. A plurality of ropes extending up to the top of the hoistway mechanically connect the car and the counterweight together. In a 1:1 roping arrangement, the ropes attach directly to the car and the counterweight. In a 2:1 roping arrangement, a sheave is mounted on the top of the car, or on the top of the counterweight, or both, to route the ropes back up the hoistway to a fixed position within the hoistway.
The car and counterweight may be driven within the hoistway by either a powered sheave or a motor driven counterweight. The powered sheave is fixed at the top of the hoistway and may or may not be used in conjunction with one or more idler sheaves. The motor driven counterweight also uses sheaves at the top of the hoistway, but the means for driving the car and counterweight is a linear motor attached to the counterweight frame.
A tension within high-rise building design is the need for effective elevatoring versus the amount of non-income producing space required by the elevators. Buildings must possess elevators having sufficient capacity and control to efficiently convey passengers at peak traffic times. The simplest way to satisfy this requirement is to design a building with excess elevator capacity to handle the peak traffic loads. Excess elevators may satisfy the peak traffic needs of the building, but they also reduce the amount of income producing space in a building and are therefore in conflict with the goal of maximizing income generating space.
One method for decreasing the building space requirements of elevators within a building without adversely effecting the capacity of the elevators is to use 2:1 roping on the counterweight of each elevator. A 2:1 roped counterweight only travels half as far as the 1:1 car to which it is connected and therefore only requires half the space normally necessary for the counterweight. A disadvantage of this method is that the area under the counterweight must be reinforced to handle the impact of the counterweight in the event of a free fall. Another method for increasing the capacity of a building's elevatoring is to employ tandem or "double-decker" elevator cars where one car is attached to the top of the other car. The theoretical capacity of an elevator shaft could be doubled with such an arrangement. Realistically, however, elevator traffic may not be best served when two cars always stop at adjacent floors. Double-decker cars are also larger in mass and therefore require a heavier counterweight and other equipment such as larger capacity safety buffers and sheaves.